There are bleacher seats with back support, but they are cumbersome to carry and if you stand up, you can knock them onto the people behind you. Also, they’re a bit anti-social in a sold out event where everyone’s squeezed in.
I’ve found inflatable bleacher cushions to be a good option. Some store very small in a pouch, then you blow them up at the game. If you tip them off the seat, nobody gets hurt. No back support, but they make sitting a lot more comfortable.